Various types of anti wheel blocking braking systems are known. Usually, such systems include transducers coupled to the wheels which provide output signals representative of speed of the wheel which are applied to suitable evaluation circuits and which process the signals, for example to obtain vehicle acceleration or deceleration signals, rate of change of acceleration/deceleration, and the like; the evaluation circuit then provides control signals applied to valves which, selectively, apply pressurized brake fluid to the wheel brake cylinders, drain pressurized brake fluid therefrom, or maintain a predetermined braking pressure by blocking both admission as well as drainage of brake fluid from the cylinders, once pressurized brake fluid has been applied thereto. Some systems of this type utilize control apparatus which provides change in braking pressure in steps, for example by pulses which, in pulsed intervals, increases the braking pressure.
Circuits and evaluation signals of this type are known, see, for example, the referenced U.S. Pat. Nos. 3,888,550 and 4,054,328. In accordance with the arrangement there disclosed, braking pressure is first applied, and, if the wheel tends to block, a relief of braking pressure is then commanded. After relief of braking pressure, that is, drop of braking pressure, a certain braking pressure may be maintained and then, again, increase gradually. The rate of increase of braking pressure, after a drop, and subsequent maintenance of braking pressure may be in accordance with a gradient or rate of change which is smaller than the initial braking pressure build-up.
Various circuits may be used to carry out the transfer functions desired between commanded braking pressure and desirable braking pressure in the light of vehicle characteristics and a predetermined program of build-up, relief, and/or maintenance of braking pressure. For example, in accordance with the disclosure of the referenced patents, a control pulse is generated which causes rapid increase of braking pressure, to be followed by a sequence of suitably arranged pulses which result in further increase of braking pressure at a lower rate, however, since there will be gaps between the pulses during which the braking pressure is maintained. In accordance with U.S. Pat. No. 3,888,550 a fixed timing interval is controlled, for example by a timing circuit which provides for a fixed time interval, which commands rapid increase of braking pressure. In U.S. Pat. No. 4,054,328, the rapid increase of braking pressure is made dependent on the degree of a prior build-up of braking pressure in a prior control cycle. The purpose, of course, is to optimize the increase in braking pressure and thus to optimize the overall braking effect to be obtained by the system.